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Full text of "Scientific Papers - Vi"

410.
ON THE ATTENUATION OF SOUND IN THE ATMOSPHERE. "
[Advisory Committee for Aeronautics.    August, 1916.]
IN T. 749, Major Taylor presents some calculations which " shew that the chief cause of the dissipation of sound during its transmission through the lower atmosphere must be sought for in the eddying motion which is known to exist there. The amount of dissipation which these calculations would lead us to expect from our knowledge of the structure of the lower atmosphere agrees, as well as the rough nature of the observations permit, with the amount of dissipation given by Mr Lindemarm."
The problem discussed is one of importance and it is attended with considerable difficulties. There can be no doubt that on many occasions, perhaps one might say normally, the attenuation is much more rapid than according to the law of inverse squares. Some 20 years ago (Scientific Papers, Vol. iv. p. 298) I calculated that according to this law the sound of a Trinity House syren, absorbing 60 horse-power, should be audible to 2700 kilometres!
A failure to propagate, so far as it is uniform on all occasions, would naturally be attributed to dissipative action. I am here using the word in th<? usual and narrower technical sense, implying a degradation of energy from the mechanical form into heat, or a passage of heat from a higher to a lower temperature. Although there must certainly be dissipation consequent upon radiation and conduction of heat, it does not appear that these causes are adequate to explain the attenuation of sound sometimes observed, even at moderate distances. This question is discussed in Phil. Mag. XLVII. p. 308, 1899 (Scientific Papers, Vol. IV. p. 376) in connexion with some observations of Wilmer Duff.
If we put dissipation out of account, the energy of a sound wave, advancing on a broad front, remains mechanical, and we have to consider what becomes of it. Part of tKe sound may be reflected, and there is no doubt at all that, whatever may be the mechanism, reflection does really occur, even when no obstacles are visible. At Sb Catherine's Point in: 1901,1 heard strong echoes
272oxovi. A, p. 205 (1901) ;   Scientific Papers, Vol. iv. p. 510. [1918.   The experiments here proposed have been skilfully carried into effect by Hartshorn, working in my son's laboratory, Proc. Roy. Soo. A, Vol. xoiv. p. 155, 1917.]f the assumption that viscosity may be neglected when a jet moves with high velocity through quiescent air.